IT probably has been posted but I don't know where.
You should remove all the old covering, if you don't it can show through sometimes or may not seal well later. If you have a heat gun you can use heat ( sparingly ) to loosen any covering that is stuck. If it won't come off with heat you will have to scrap it off ( carefully ) or sand it off. I won't suggest any chemicals as they might keep the new covering from sticking.
If you have any oil soaking, you need to take care of that also. There are a lot of post dealing with this, they refer to K2r, kitty litter and baking soda for a few. Good Luck !! ENJOY !!! RED